This project concerns ESR application in studies of carcinogenic compounds and in nucleic acid research. The program can be classified into two areas: 1) The investigation of radical formation of polycyclic hydrocarbons by chemical induction or by enzymic oxidation as related to their carcinogenicities. In the last year, ESR studies on iodine-induced radicals of benz (a) anthracene series have been evaluated. Formation of the 6-oxobenzo (a) pyrene radical form benz (a) pyrene (B (a) P) upon incubation in liver homogenate has been measured quantitatively by the ERS technique. The % of B (a) P metabolized to 6-OH-B(a)P is estimted to be not less than 5% and probably about 20-30%. This experiment for the first time has identified that the formation of 6-OH-B(a)P and 6-Oxo-B-(a)P must be an important pathway of B(a)P metabolism of liver. Molecular oxygen has been shown to be required for the formation of the oxo-radical and the quinone from 6-OH-B(a)P. The data suggest that the 6-oxo-B(a)P radical is unlikely to be an obligatory intermediate in the spontaneous oxidation of 6-OH-B(a)P to quinone, but is a good candidate as a reactive species in the pathway of binding to nucleic acids. 2) The investigation of spin-labelled nucleic acid. In the coming year spin-labelled nucleic acid encapsulated by plant virus into a linear form or a globular form will be investigated in comparison to their free state. In addition, the fate of the spin-labelled I plus C exposed to mammalian cells will be followed by the ESR technique. The above experiment prepares the foundation for the study of nucleic acid-protein interactions and nucleic acid-cell interactions using spin label as a probe.